Marcia Jose Sanchez / APHigh gas prices are advertised at a gas station in Half Moon Bay, Calif., Tuesday, May 27, 2008. Retail gas prices jumped a cent Friday to a new national average over $3.96 a gallon, putting them on a course to hit $4 by early next week. Oil futures, meanwhile, traded in a narrow range as investors tried to determine whether recent price declines were temporary.

Some things will stretch gas dollars; some won't. Steve Mazor runs the Automobile Club of Southern California's Automotive Research Center and put tips to the test to see whether they are myth or truth. A few major myths and a couple of tips:

MYTH: Oil companies run up prices to cash in on Sunday drivers, so the best time to fill up is Wednesday.

TRUTH: Just because that day is farthest from the weekend doesn't mean drivers will save at the pump. "Really, prices are being dictated regionally, so I don't think that plays a role anymore," Mazor said.

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MYTH: Morning is the best time of day to pump gas because gas is denser when it is cooler.

TRUTH: It is true that gas is denser when it's colder, but double-walled underground gas tanks retain the temperature of the gas at the time it was pumped underground. To meet demand, tankers pump gas underground around the clock.

"We sell gas so quickly that if it was hot when it went in, it will be hot when it comes out," Mazor said. The difference may amount to "a few cents a tank."

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MYTH: A clean air filter makes a difference.

TRUTH: A few cars had slightly better fuel economy when dirty air filters were replaced, but when cars were tested with blocked air filters, fuel economy "didn't get worse, as we expected," Mazor said.

In fact, some cars slightly improved fuel economy with dirty air filters, but "you make your car a polluter for maybe 1 percent better fuel economy."

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MYTH: Gizmos or additives like acetone will save gallons.

TRUTH: These products don't help even a little bit, Mazor said.

"We have tested many fuel additives and products ... or devices that go on the battery cable, and none of them have ever shown any measurable fuel economy improvement," Mazor said.

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MYTH: It's better to idle than turn off your engine.

TRUTH: "If you're going to sit for more than about 90 seconds, go ahead and turn it off," Mazor said. It's also a good idea to avoid drive-thrus when visiting a fast food restaurant. "Try a novel idea; park your car and go inside," Mazor said. And in a drive-through, "you're going no distance at all; the longer you idle, the more gas you use."

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MYTH: It's always best to sweat it out without air-conditioning.

TRUTH: This is true for vehicles that were made before 1980, but for modern cars, "it's a relatively small effect" because the efficiency of air conditioning systems has "improved dramatically," Mazor said.

"The rule of thumb that we tell people is if you're going below 45 mph, roll the windows down and turn the A/C off. But if you're on the freeway, going faster, roll up your windows and use A/C," Mazor said. He warns that the effect is not dramatic, but is a good habit that could save 5 percent on fuel economy.

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ALSO: Check your tires. Be more considerate, stay calm.

Drivers can lose as much as 2 percent of their fuel efficiency for every pound their tires are underinflated.

And slamming on the brakes is the worst way to get around. "You just want to slow down a little, reduce your weight of acceleration; you do not want to hotrod," Mazor said.